It has been demanded in the industrial field that the techniques for rolling a hard material or a material difficult to be processed or a thin plate with a reduced rolling load are accomplished by using work rolls of a miniaturized diameter. To meet this demand, a 20-high rolling mill called a Sendzimir mill and disclosed in U.S. Pat. No. 2,776,580 has long been used, in which, however, it is difficult to set the crown and shape of a plate material to be rolled to predetermined permissible levels. In the case of a rolling mill which is obtained by improving a conventional 4-high rolling mill, and which employs small-diameter work rolls, a backup roll driving system is necessarily adopted. Due to this driving system, a tangential force is applied from the backup rolls to the work rolls, so that the axial flexure of the work rolls occurs in the lateral direction. In order to prevent this axial flexure, various types of rolling mills have been developed, which include a so-called MKW mill (refer to U.S. Pat. No. 4,598,566), a support roller-carrying 5-high mill (refer to U.S. Pat. Nos. 4,577,480 and 4,539,834), and a support roller-carrying 6-high mill (refer to U.S. Pat. Nos. 4,270,377; 4,563,888 and 4,531,394), in all of which a primary rolling load is supported by unitarily-formed backup rolls with support rollers provided substantially in the same plane as the work rolls so as to prevent the lateral flexure of the work rolls, i.e. the axial flexure thereof in the direction of the path of a material to be rolled.
These various types of rolling mills are divided into two groups depending upon the support systems employed therein for supporting the small-diameter work rolls by the support rollers disposed along the path of the material to be rolled.
(i) Rolling mills in which the work rolls are supported by support rollers with the work rolls and support rollers arranged so that the centers of the side surfaces thereof are on one straight line (refer to U.S. Pat. Nos. 4,598,566; 4,577,480 and 4,563,888).
(ii) Rolling mills in which a plurality of rows of support rollers are provided, by which the idle rollers provided between the work rolls and support rollers are stably supported (refer to U.S. Pat. Nos. 4,270,377 and 4,531,394).
In the work roll supporting system in the rolling mills in (i), the work rolls, idle rollers and support rollers are arranged in the mentioned order so that the centers of the the side surfaces thereof are substantially on one straight line. If the centers of the side surfaces of the work rolls, idle rollers and support rollers in the support system in (i) are out of a straight line connecting these centers, a bending force based on the force generated by the work rolls is exerted, especially, on the idle rollers. Therefore, it becomes necessary that the diameter of the idle rollers be increased to a certain extent, and, due to such dynamic restrictions, the diameter of the work rolls cannot be reduced, either.
In the work roll supporting system in (ii), the support rollers supporting the idle rollers, which are disposed in contact with and in substantially the same plane as the work rolls, are provided in a plurality of rows, and the force generated by the work rolls can be supported in a dynamically stabilized state. However, a space for holding two support rollers is required, so that the extent to which the diameter of the work rolls can be reduced is limited.
The present applicant developed 6 high rolling mills having a greatly-improved capability of controlling the crown and shape of a plate type material to be rolled, as disclosed in U.S. Pat. Nos. 3,818,743 and 4,369,646. In these rolling mills, axially-shiftable intermediate rolls are provided between the backup rolls and work rolls, and bending means on the work rolls or the work rolls and intermediate rolls, for the purpose of improving the capability of controlling the shape of the material to be rolled, an intermediate roll shifting operation and a roll bending action being suitably combined.
The present applicant also proposed a rolling mill, which is disclosed in U.S. Pat. No. 4,614,103, and which is provided with horizontal support rollers and horizontal backup rollers as means for reducing the flexure, which poses problems when the diameter of the work rolls in the 6 high rolling mill disclosed in U.S. Pat. No. 4,369,646 is further reduced, of the work rolls in the direction of the path of the material, which support rollers and backup rollers are arranged so that the centers of the side surfaces thereof are on one straight line, a pre-stress being applied to the work rolls via a roller-supporting frame. The present applicant also filed as a prior application an application directed to a multihigh rolling mill which is obtained by improving the above-described multihigh rolling mill in which the horizontal support rollers and horizontal backup rollers, which support the small-diameter work rolls, are arranged so that the centers of the side surfaces thereof are on one straight line. The multihigh rolling mill in this prior application is constructed by making pivotable the frame, which support these rollers, and engaging the this frame with a hydraulic cylinder, in such a manner that a vertical bending force can be applied effectively to the small diameter work rolls. (U.S. patent application Ser. No. 847,489 corresponding to German Patent Laid-open No. DE-OS 3,610,889) It has been demanded in the industrial world that, when an extremely thin material of not more than 0.2 mm in thickness or a thin hard plate, such as a stainless steel plate is rolled, work rolls the diameter of which is reduced to as great an extent as possible be used so as to set the rolling reduction to as high a level as possible. However, the results of the analyses and researches conducted by the present inventors show that, when work rolls and intermediate rolls of reduced diameters are used in the rolling of a work of, for example, 1300 mm in width, the end portions of the work which have a width corresponding to about 1/4 of a total width thereof are curved upward and downward, i.e., the so-called quarter buckling occurs in these end portions of the work, so that the surface of the work is waved. When the quarter buckling has once occurred in a work, it is considerably difficult to remove the same, and various troubles occur in the later processing steps. Therefore, in order to reduce the diameter of the work rolls, it is necessary that the diameter of the intermediate rolls be increased to a certain extent. However, increasing the diameter of the intermediate rolls has the following problems. In the multihigh rolling mills disclosed in U.S. Pat. Nos. 4,270,377 and 4,531,394, which are provided with a plurality of rows of support rollers, an intermediate roll 6 is provided between a small-diameter work roll 5 and a backup roll 7, and intermediate rollers 4 on both sides, in the horizontal direction, of the small-diameter work roll 5, each of which intermediate rollers 4 is supported on upper and lower support rollers 13, 14, as shown in, for example, FIG. 6a out of FIGS. 6a-6c. Thus, the intermediate rollers 4 are disposed stably without interfering with the intermediate rolls 6, so that the lateral (horizontal) movement of the work roll 5 is restricted sufficiently. However, when the diameter of the work roll 5 D.sub.Wb is further reduced in comparison to D.sub.Wa, shown in FIG. 6a, and the diameter of the intermediate roll 6 D.sub.Ib is increased as compared with that of the intermediate roll of FIG. 6a D.sub.Ia, as shown in FIG. 6b, the upper support roller 13 and intermediate roll 6 interfere with each other, so that this structure does not function as a rolling mill.